Horizontal pianos (grand and table) and upright pianos are known. These pianos differ substantially in that part regarding the action mechanism; in horizontal pianos the hammers move from the bottom upwards, whereas in upright pianos they move forwards and backwards.
FIG. 1 shows a schematic lateral view of an upright piano action mechanism of the state of the art, illustrating inter alia those elements lying within the dashed-line area, i.e. the hammer 22, the shank 20, the hammer butt 18 and the balance hammer 19, all pivoted on the pin 21 and rigidly connected together into a single lever-hammer 3. Of these elements, the hammer has a movement constantly close to the horizontal line, whereas the hammer butt 18 and balance hammer 19 move close to the vertical. However, given that the weight of the hammer is preponderant, the center of gravity of this lever-hammer advances towards the string 1 during playing until it exceeds the vertical through the pin 21. The action of the lever becomes counter-productive, this signifying that at a certain moment the resistance becomes zero and hence the hammer disappears from the pianist's perception. All that remains, in reality, is a resistance due to other parts of the action mechanism (key 2 and wippen 10) and in particular to the energy of the springs, this creating an artificial situation, totally different from that of the grand piano. In this respect, in a grand piano the resistance perceived by the pianist is due to an angular momentum of the hammer which is greater than that of the upright piano, and in particular constant until reaching the string, whereas in the upright piano the angular momentum of the lever-hammer 3, already modest from the start, rapidly decreases to become negative. This determines total loss of control of the hammer at the moment of striking and hence loss of touch, i.e. of the ability to influence the tone color and expressive characteristics of the sound, which can be decided only at that moment.
Physically, the touch consists of determining the attack transient, i.e. in that apparently chaotic stage which precedes the stationary wave. In the piano the stationary wave comes in the continuation of the sound after striking, the pianist being unable to directly intervene in this continuation. Consequently, determining the attack transient by controlling the modalities of encounter of the hammer with the string is all that the pianist can do to influence the sound quality.
In the upright piano the only one of these modalities which can be decided at the hammer departure is the sound intensity, hence the pianist is only able to control the vibration amplitude, which is determined by the initial launch energy.
To this must be added the fact that the facility to control the sound is further diminished by the action of the springs (the spring 28 of the hammer butt 18 and the spring 27 of the damper 26 in FIG. 1), the dynamics of which cannot be modified at the act of the musical execution. Of these, the spring 28 of the hammer butt 18, which ensures return of the hammer 22, interferes directly on the hammer stroke until it entirely replaces the nullified resistance with its own energy, its negative effect on touch control being hence evident. On the other hand the spring 27 of the damper 26 acts on an intermediate lever of the action mechanism, i.e. the wippen 10. However this is more energetic than the other, its negative effect on the touch (presumably not less than that of the spring 28) manifesting itself particularly as a closure of the sound (extinction transient), which is totally uncontrollable by the pianist.
Hence for clarity the touch must also be defined from a subjective viewpoint, i.e. in the perception of the pianist. Touch control is a feedback process, i.e. a certain muscular action determines a certain sound effect, this influencing in real time the next muscular action, and so on until an automatic process is created, constituting one of the fundamentals of the pianist's technique, i.e. the capacity to give musical meanings to the sound. However this process starts only from a certain threshold (i.e. from a minimum perceptive level), and it can be considered that this threshold is a substantially objective detail, dependent only in certain cases, and only partially, on subjective situations, and that to attain it certain objective physical conditions are necessary which require particular characteristics of the instrument. This means that if the pianist's fingers do not “sense” the hammer because of the limits of the instrument, the ear cannot hear a variation in timbre such as to influence the motory action. Below a threshold defined in this manner, evidently no feedback is possible, which objectively means that the instrument, as it does not possess it, does not enable touch.
This threshold is natural in a grand piano, given that the angular momentum of the hammer is sufficiently high and, as already stated, substantially constant, but in an upright piano it appears to be unattainable. However a solution to this problem exists, based on utilizing a “lens effect”, a perceptive phenomenon by virtue of which the pianist can continue to sense the initial resistance of the lever-hammer until reaching the string or, perhaps more exactly, perceives this resistance as if the angular momentum of the hammer at the moment of striking were the same as that at the start. As a particular study of the causes of this phenomenon has not been possible, it can be assumed that it depends on the reaction time of our perceptions which, both for that regarding hearing (and also sight) and for that regarding the grand musculature, is generally defined as 75/1000 of a second. Hence, touch in an upright piano is probably possible only when the stroke of the hammer 22 has a duration of less than that time, hence making separate perception of the initial angular momentum of the stroke of the lever-hammer 3 from the final momentum impossible, if the final momentum is positive and reaches a perceptible minimum.